The present invention relates to ink jet printing and more particularly to a continuous ink jet printer with reduced solvent consumption.
In ink jet printing systems the print is made up of individual droplets of ink generated at a nozzle and propelled towards a substrate. There are two principal systems: drop on demand where ink droplets for printing are generated as and when required; and continuous ink jet printing in which droplets are continuously produced and only selected ones are directed towards the substrate, the others being recirculated to an ink supply.
Continuous ink jet printers supply pressurized ink to a print head assembly having a drop generator where a continuous stream of ink emanating from a nozzle is broken up into individual regular drops by an oscillating piezoelectric element. The drops are directed past a charge electrode where they are selectively and separately given a predetermined charge before passing through a transverse electric field provided across a pair of deflection plates. Each charged drop is deflected by the field by an amount that is dependent on its charge magnitude before impinging on the substrate whereas the uncharged drops proceed without deflection and are collected at a gutter from where they are recirculated to the ink supply for reuse. A phase measurement system is also usually present as part of deflection plate assembly and is used to ensure synchronization of deflection for the droplets. The charged drops bypass the gutter and hit the substrate at a position determined by the charge on the drop and the position of the substrate relative to the print head assembly. Typically the substrate is moved relative to the print head assembly in one direction and the drops are deflected in a direction generally perpendicular thereto, although the deflection plates may be oriented at an inclination to the perpendicular to compensate for the speed of the substrate (the movement of the substrate relative to the print head assembly between drops arriving means that a line of drops would otherwise not quite extend perpendicularly to the direction of movement of the substrate).
In continuous ink jet printing a character is printed from a matrix comprising a regular array of potential drop positions. Each matrix comprises a plurality of columns (strokes), each being defined by a line comprising a plurality of potential drop positions (e.g. seven) determined by the charge applied to the drops. Thus each usable drop is charged according to its intended position in the stroke. If a particular drop is not to be used then the drop is not charged and it is captured at the gutter for recirculation. This cycle repeats for all strokes in a matrix and then starts again for the next character matrix.
Ink is delivered under pressure to the print head assembly from an ink supply system that is generally housed within a sealed compartment of a cabinet that includes a separate compartment for control circuitry and a user interface panel. The system includes a main pump that draws the ink from a reservoir or tank via a filter and delivers it under pressure to the print head assembly. As ink is consumed the tank is refilled as necessary from an ink source such as a replaceable ink cartridge that is releasably connected to the tank by a supply conduit. The ink is fed from the tank via a flexible delivery conduit to the print head assembly. Electrical power to operate the heater in the print head assembly and the drop generator are supplied by power supply system cables, typically forming part of the supply conduit. The unused ink drops captured by the gutter are recirculated to the tank via a return conduit, typically located as part of the supply conduit, by a pump. The flow of ink in each of the conduits is generally controlled by solenoid valves and/or other like components.
As the ink circulates through the system, there is a tendency for it to thicken as a result of solvent evaporation. This is particularly a problem in relation to the recirculated ink that has been exposed to air in its passage between the nozzle and the gutter. Air that is used to clear the gutter has to be exhausted to atmosphere carrying solvent with it. To compensate for this “make-up” solvent is added to the ink as required from a solvent source such as a replaceable solvent cartridge so as to maintain the ink viscosity within desired limits when the ink is at the correct operating temperature. The loss of solvent due to evaporation increases the cost of operating the printer (due to the need to supply makeup solvent) and also can create undesirable odors in the area around the printer. The solvent may also be used for flushing components of the print head assembly, such as the nozzle and the gutter, in a cleaning cycle.